Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVEMENTS IN OR RELATING TO OPTICAL COMBINERS
The present invention relates to a combiner and a method of forming a
combiner.
A combiner is conventionally formed from a first glass prism having a surface
arranged
to carry a dielectric coating applied to the surface, such that the dielectric
coating is
arranged to reflect light from an image, and a second glass prism having a
surface bonded
to the surface of the first glass prism using optical cement. In this manner,
when a user,
typically a pilot of an aircraft, views a scene through the cornbiner, the
image appears to
be superimposed on the scene. It will be understood that an image generator,
such as a
cathode ray tube, flat panel display or another suitable image generating
means, can
generate an image containing information such as primary flight data relating
to the
aircraft, for example, navigation and guidance information or cues for the
user to follow.
Therefore, the image can be conveyed to the user so as to overlay a scene
viewed by the
user through the combiner such that the user does not need to look away from
the scene,
for example to read primary flight data from a head down display, and the
image is
provided at infinity such that the eye of the user does not need to be
refocused in order
to view the image.
However, if the combiner is to be used as part of or an attachment to
headgear, such as
a helmet or goggles, worn by the user then the weight of the combiner will
increase the
mass required to be supported by the head and particularly the neck of the
user, thereby
increasing user fatigue when using the headgear. Furthermore, the safety of
the user can
also be compromised, particularly during ejection from the aircraft.
Accordingly, the
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headgear or a part thereof needs to be removed prior to ejection of the user
from the
aircraft.
The weight of the combiner can be reduced by replacing the first and second
glass prisms
S with a pair of plastic prisms formed from one of a range of optical
plastics. Such plastics
have been used to form lenses in optical systems for many years where light
weight,
safety or cost effectiveness of a mass produced lens is desirable. However,
these plastics
are relatively unstable materials to polish or to manufacture to a high
precision, for
example by moulding, due to their physical properties. When such plastics are
polished
or moulded they deform under manufacturing stress which causes the finished
surface of
the prism to differ to that original intended. Furthermore, the surface of the
plastic is soft
making a smooth polished finish difficult to achieve.
Furthermore, perhaps more importantly, without special techniques, which are
still under
development, an optical coating applied to a plastic surface is of a much
poorer quality
than an optical coating applied to a glass surface. That is the optical
coating tends to
craze thereby hazing the user's vision of a scene viewed through the combiner
or the
optical coating may peel from the plastic surface.
It is an object of the present invention to obviate or mitigate the
disadvantages associated
with the prior art.
According to a first aspect of the invention, a combiner comprises a glass
reflector having
a surface, a plastic prism having a surface arranged to adhere to the surface
of the glass
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reflector, and a bonding agent arranged to adhere the surface of the glass
reflector to the
surface of the prism, wherein the bonding agent is arranged to allow the glass
reflector
and the plastic prism to expand and contract at different rates with respect
to one another.
In this manner, the different rates of expansion and contraction, that is the
difference in
the expansion coefficient between the glass reflector and the plastic prism
can be
absorbed by the bonding agent used to adhere the glass reflector to the
plastic prism
during thermal cycling.
The surface of the glass reflector may be curved and the surface of the
plastic prism may
be arranged to complement the surface of the glass reflector. The surface of
the glass
reflector may also comprise a dielectric coating. In this manner, an optical
coating can
be disposed at the surface of the glass reflector thereby inhibiting crazing
associated with
disposing an optical coating on a surface of a plastic prism.
Preferably, the glass reflector may have a second surface arranged to adhere
to a surface
of a second plastic prism, and a bonding agent may be arranged to adhere the
second
surface of the glass reflector to the surface of the second plastic prism. The
second
surface of the glass reflector may also comprise a dielectric coating. The
second surface
of the glass reflector may be curved and the surface of the second plastic
prism may be
arranged to complement the second surface of the glass reflector.
Alternatively, the plastic prism may have a second surface arranged to adhere
to a surface
of a second glass reflector, and a bonding agent may be arranged to adhere the
second
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surface of the plastic prism to the surface of the second glass reflector. The
surface of
the second glass reflector may also comprise a dielectric coating. The surface
of the
second glass reflector may be curved and the second surface of the plastic
prism may be
arranged to complement the surface of the second glass reflector.
The or each glass reflector may be a pellicle reflector. The bonding agent may
be a
flexible silicone adhesive.
Preferably, the combiner may be an eyepiece combiner arranged to form part of
a helmet
t 0 mounted display system. A combiner according to the invention is lighter
than a
conventional combiner formed from glass incorporate within headgear to be worn
by a
user, therefore the combiner reduces the loading of the headgear on the neck
of the user
and the user may eject from the aircraft while wearing the headgear
incorporating the
combiner.
l5
According to a second aspect of the invention, as method of forming a combiner
includes
forming a surface on a glass reflector, forming a surface on a plastic prism
and arranging
the surface to adhere to the surface of the glass reflector, and bonding the
surface of the
glass reflector to the surface of the prism, such that the bonding agent is
arranged to allow
2,0 the glass reflector and the plastic prism to expand and contract at
different rates with
respect to one another.
The method may include priming the surface of the glass reflector and the
surface of the
plastic prism prior to bonding the surfaces to one another. The method may
include
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coating the surface of the glass reflector with a dielectric coating prior to
priming the
surface of the glass reflector.
Preferably, the method may include forming a second surface on the glass
reflector,
5 forming a surface on a second plastic prism and bonding the second surface
of the glass
reflector to the surface of the second plastic prism. The method may include
priming the
second surface of the glass reflector and the surface of the second plastic
prism prior to
bonding the surfaces to one another. The method may include coating the second
surface
of the glass reflector with a dielectric coating prior to priming the second
surface of the
l0 glass reflector.
Alternatively, the method may include forming a second surface on the plastic
prism,
forming a surface on a second glass reflector and bonding the second surface
of the
plastic prism to the surface of the second glass reflector. The method may
include
priming the second surface of the plastic prism and the surface of the second
glass
reflector prior to bonding the surfaces to one another. The method may include
coating
the surface of the second glass reflector with a dielectric coating prior to
priming the
surface of the second glass reflector.
The method may include forming the or each surfaces such that it is curved.
The invention will now be described, by way of example only, with reference to
the
accompanying drawings, in which:
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Figure 1 illustrates in cross section a combiner according to a first
embodiment of the
invention;
Figure 2 illustrates in cross section a combiner according to a second
embodiment of the
i invention, and
Figure 3 schematically illustrates a combiner as shown in Figure 1 used as
part of a head
up display system.
The inventor has realised that the difference in quality of an optical coating
on a plastic
surface as opposed to a glass surface is due to the dielectric material used
to form the
optical coating. Dielectric materials, which are applied to the plastic
surface in a number
of thin layers to constitute the optical coating, have similar physical
properties to those
of glass and thus there exists a large difference in thermal expansion
coefficient between
the dielectric materials used to form the optical coating and the thermal
expansion
coefficient of the plastic surface. This leads to a crazing effect of the
optical coating after
thermally cycling which effects a user's vision of a scene viewed through the
plastic
surface and the optical coating.
0 Furthermore, the inventor has also realised that a plastic surface has a
lower melting point
compared with a glass surface and thus the deposition of an optical coating on
to a plastic
surface must be done at a substantially lower temperature compared with
deposition of
the optical coating on a glass surface. This results in a poorer adhesion of
the optical
coating to the plastic surface than can be achieved between an optical coating
and a glass
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surface.
In Figure 1, a first embodiment of the invention comprises a combiner 10
having a glass
reflector 11, such as a glass pellicle reflector, that has first and second
curved surfaces
12 and 13 that have an optical coating, not illustrated, on either or both of
the surfaces
12, 13. A first plastic prism 14 has a surface 15 arranged to complement the
curved
surface 12 of the glass reflector 11. A second plastic prism 16 has a surface
17 arranged
to complement the second curved surface 13 of the glass reflector 11. The
first and
second curved surfaces 12 and 13 and the surfaces 15 and 17 of the first and
second
l0 plastic prisms 14 and 16 are each primed with a suitable primer which is
allowed to dry
and is then polished.
The glass reflector I 1, first plastic prism 14 and second plastic prism 16
are arranged
with respect to one another such that the first curved surface 12 of the glass
reflector 11
t 5 forms a well 18 with the surface 15 of the first plastic prism 14.
Similarly, the second
curved surface 13 of the glass reflector 11 forms a well 19 with the surface
17 of the
second plastic prism 16.
It will be understood that the glass reflector I 1, first plastic prism 14 and
second plastic
ZO prism 16 can be retained in position relative to one another using a jig,
not illustrated.
The wells 18 and 19 are completed with a seal 20 arranged to close the wells
19 and 20
at a lower end 21. That is, the gap between the first curved surface 12 of the
glass
reflector and surface I 5 of the first plastic prism 14 and the second curved
surface 13 of
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the glass reflector 1 l and surface 17 of the second plastic prism 16 are
sealed at the lower
end 21. The seal 20 may be formed from a flexible silicone material.
A bonding agent 22, represented by double hatchings in Figure 1, is then
injected into the
E
wells 18 and 19 so as to fill the wells 18 and 19. The combiner 10 is then
placed in a
vacuum chamber to remove any trapped air bubbles within the wells 18 and 19.
The
bonding agent 22, 23 is then allowed to cure at a temperature which will not
unduly stress
the glass reflector 11, first plastic prism 14 and second plastic prism 16.
It will be understood, that a dielectric coating, not illustrated, can be
applied to either the
first curved surface 12 or second curved surface 13, or indeed both surfaces
12 and 13,
prior to the surfaces 12 and 13 being primed and the glass reflector 11 being
adhered to
the first and second plastic prisms 14 and 16. It will be understood that the
optical
coating has both light transmissive and light reflective properties. In this
manner, a
combiner 10, suitable for use as an eyepiece in a head up display, can be
formed from
materials lighter than a conventional combiner using a pair of glass prisms
bonded to one
another.
The bonding agent 22, 23 is formed from a flexible silicone material arranged
to
compensate for the different rates of expansion and contractions, that is the
difference in
the expansion coefficient between the glass reflector 11 and the first and
second plastic
prisms 12 and 16. Therefore, when the combiner 10 is caused to expand or
contract
during thermal cycling the adhesive will absorb shear stress caused by the
different
expansion coefficients, thereby providing a means to suitably adhere the glass
reflector
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11 to the first and second and plastic prisms 14 and l6: - Furthermore, the
crazing
associated with prior art optical coating on a plastic surface is obviated as
the optical
coating is applied to a glass surface of the glass reflector 11. Furthermore,
the glass
reflector 11 will have a greater range of reflective indices and dispersive
properties
compared with a plastic surface having the same optical coating.
It will be understood that the first curved surface 12 may be simply curved or
may be
concaved, that is substantially spherical. Similarly, the second curved
surface 13 may be
simply curved to complement the first curved surface 12 or may be convex, that
is
substantially spherical, to complement the first curved surface 12.
The second plastic prism 16 tends to distort light passing through its body,
thereby
causing distortion of the scene as viewed by a user. Therefore, the first
plastic prism 14
is arranged to correct distortions generated by the second plastic prison 16.
In Figure 2, a second embodiment of the invention, comprises a combiner 30
having a
plastic prism 31 which has a first curved surface 32 and a second curved
surface 33. A
first glass reflector 34 has a surface 35 arranged to complement the first
curved surface
32 of the plastic prism 31 and a second glass reflector 36 having a surface 37
arranged
to complement the second curved surface 33 of the plastic prism 31. The first
and second
curved surfaces 32 and 37 of the plastics prism 31 and the surfaces 35 and 37
respectively
of the first and second reflectors 34 and 36 are each primed with a suitable
primer which
is allowed to dry and is then polished. The first and second glass reflectors
34 and 36 can
be glass pellicle reflectors.
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The first glass reflector 34 is arranged with respect to the plastic prism 31
such that the
first curved surface 32 and surface 35 of the first glass reflector 34 form a
well 38 and a
seal 39 closes an end of the well 38. The seal 39 can be formed from a
flexible silicone
material.
5
Similarly, the second glass reflector 36 and plastic prism 31 may be arranged
with respect
to one another such that the first curved surface 32 and surface 37 of second
glass
reflector 36 form a well 40 closed at one end with a seal 41. Again, the seal
41 can be
formed from a flexible silicone material.
~o
It will be understood, that the plastic prism 31 and the first and second
glass reflectors
34 and 36 may be held with respect to one another using a jig, not
illustrated.
Bonding agent 42 and 43, represented by double cross hatching in Figure 2, is
then
injected into the wells 38 and 40. As in the embodiment illustrated in Figure
l, the
combiner 30 is placed in a vacuum chamber so as to remove any air bubbles in
the
bonding agent 42, 43 and the bonding agent 42, 43 is allowed to cure at a
temperature
which does not unduly stress the plastic prism 31 and the first and second
glass reflectors
34 and 36.
In this embodiment, the plastic prism 31 has extensions 44 and 45 to provide a
suitable
mounting position for the combiner 30 within an optical system, not
illustrated.
It will be understood that the first and second curved surfaces 32 and 33 of
the prism 31
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may be curved or convex, that is substantially spherical, and that the
surfaces 35 and 37
respectively of the first and second glass reflectors 34 and 36 can be
concaved to
complement their respective curved surface 32, 33.
The second glass reflector 36 tends to distort light passing through its body,
thereby
causing distortion of the scene as viewed by a user. Therefore, the first
glass reflector 34
is arranged to correct distortions generated by the second glass reflector 36.
In Figure 3, in which like references have been used to indicate similar
integers to those
LO used in Figure 1, a combiner 10 is positioned within a line of sight 50 of
an eye 51 of a
user, not illustrated. The eye 51 views a scene 52, in this instance indicated
by an arrow,
through the combiner 10. An image, not illustrated, formed by an image
generator and
optical system associated with the combiner 10, also not illustrated, follows
a light path
53 and the image is internally reflected from an aft surface 54, of the first
plastic prism
14, into the first curved surface 12 of the glass reflector 10. The optical
coating disposed
on the first curved surface 12 then reflects the image to follow a light ray
path 55 along
the line of sight 50 of the eye 51. Accordingly, the user of the combiner 10,
typically a
pilot of an aircraft, views the scene 52 through the combiner 10, that is
through the
optical coating, and the image appears to be superimposed on the scene 52.
Therefore,
an image, which can represent primary flight data relating to the aircraft,
can be conveyed
to an eye 51 of the user so as to overlay a scene 52 viewed by the eye 51
through the
combiner 10 such that the user does not need to look away from the scene 52,
for
example to read primary flight data from a head down display, and the image is
provided
at infinity such that the eye 51 of the user does not need to be refocused in
order to view
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the image. It will be understood that the combiner 30 illustrated in Figure 2
operates in
a similar manner to that described with reference to Figures l and 3.
It will also be understood that although the invention is described and
predominately
intended for use as a combiner, especially as an eyepiece combiner, to convey
information to a pilot, the invention can find applications in other
environments where
information needs to be conveyed to a user such that the user does not need to
look away
from a scene viewed through the combiner. For example, a combiner according to
the
invention could be used in other transportation vehicles and to convey
information from
a central location to a user.
